Acacia is a large genus comprising more than 1000 species belonging to the family Leguminosae, subfamily Mimosoideae. They are distributed in the warm and drier regions of the world mainly in the tropics and subtropics and are more prevalent in Australia and Africa. The genus Acacia is divided into 3 subgenera based mainly on the leaf morphology.

Subgenus : Acacia : Leaves bipinnate with stipular spines which are often swollen and inhabited by ants.

sHowever, based on the nature of leaves in mature trees, the genus Acacia can be divided into two ‘popular’ groups ; the typical bipinnate leaved species and the other with phyllodes. Phyllode is a modified petiole expanded into a leaflike structure with no leaflets, an adaptation to xerophytic conditions. The typical bipinnate leaved species are mostly found throughout the tropics, whereas the phyllodenous species occur mostly in Australia. More than 40 species have been reported in India. Gamble in his ‘Flora of Madras Presidency ' listed 23 native species for southern India and 15 of them are from Tamil Nadu. Since then, many species have been introduced and around 40 species are now found in Tamil Nadu itself. The indigenous species are mostly thorny trees or shrubs and a few are thorny stragglers like A. caesia, A. pennata and A. sinuata. Acacias occur mostly in the dry and arid regions, where the forests are often of the nature of open thorny shrubs. Many of the species have been introduced from Africa and Australia. Among the notable ones are "wattle trees" i.e. Acacia mearnsii, A. picnantha and A. dealbata, which have bipinnate leaves; and A. auriculiformis, A. holoserecia and A. mangium among the phyllodenous species. The wattle trees are almost naturalised in Ooty and Kodaikanal Hills and the black wattle is cultivated extensively on a commercial scale often to the detriment of native forest vegetation.

Acacias are established as very important economic plants since early times as source of tannins, gums, timber, fuel and fodder. They have significant pharmacological and toxicological effects. Tannins, mostly from bark, are extensively used for tanning hides and skins. Some barks are used for flavouring local spirits. Some indigenous species like A. sinuata also yield saponins used traditionally as native soap for washing. Flowers of some species are fragrant. Cassie perfume is obtained from Acacia ferrugenea. The heartwood of many acacias are used for making agricultural implements and also as firewood and are grown near villages for these purposes. Acacia gums find extensive use in medicine and confectionary and as sizing and finishing materials in the textile industry. Lac insects can be grown on several species (A. nilotica, A. catechu). Some species have been used for afforestation of wastelands like A. nilotica which can withstand some water inundation and a few such areas have become bird sanctuaries. Koondakulam in Tirunlelveli is an example.

A brief description and rough diagram (not to scale) of ten species of bipinnately leaved acacias which are commonly found in South India are given in this article.